Bisphenol A is an important chemical product as a base material of polycarbonate resins, epoxy resins and such, and is produced industrially on a large scale through the reaction of phenol and acetone. For isolating bisphenol A from the reaction mixture, the reaction mixture is first concentrated and then cooled to let bisphenol A crystallize as an adduct with phenol, and the adduct is subjected to a suitable solid/liquid separating means such as centrifuge to separate the crystals from the mother liquor.
This bisphenol A/phenol adduct is then melted and distilled to separate it into bisphenol A and phenol. Since bisphenol A tends to be thermally decomposed and transformed into isopropenylphenol or such when exposed to a high temperature, the distillation of the bipshenol A/phenol adduct for separation is usually carried out under reduced pressure. For instance, in Japanese Patent Application Laid-Open (KOKAI) No. 5-294874, there is described that the bisphenol A/phenol adduct is melted and supplied to a centrifugal thin film evaporator and phenol is evaporated away under reduced pressure and a pressure of not lower than 45 Torr. However, in order to remove phenol sufficiently without much raising the degree of vacuum, it needs to elevate the operating temperature, which involves the possibility of causing thermal decomposition of bisphenol A.
In Japanese Patent Application Laid-Open (KOKAI) No. 6-107579, there is described a technique in which a melt of a bisphenol A/phenol adduct is supplied to a flash evaporator and separated into a gaseous phase comprising phenol and a liquid phase where phenol is not more than 4% by weight, and this liquid phase is further supplied to a centrifugal thin film evaporator to evaporate away phenol. However, for reducing the phenol content in the liquid phase below 4% by weight by flash evaporation, it is necessary to evaporate a large amount of phenol, for which flash evaporation needs to be carried out under a high degree of vacuum or at a high temperature. However, flash evaporation under a high degree of vacuum may cause separating of bisphenol A or the bisphenol A/phenol adduct as a solid because of excessive drop of temperature. Also, when flash evaporation is carried out at a high temperature, bisphenol A may be decomposed when heated to produce isopropenylphenol or such, resulting in a deteriorated hue of obtained bisphenol A.
Japanese Patent Application Laid-Open (KOKAI) No. 63-275539 teaches that when bisphenol A is recovered from the bottom of a distillation column, part of the bottom product is circulated and fed back into the said distillation column along with the crystals or melt, or their mixture, of the said adduct, and also states that it is preferable to heat the circulating liquid by an amount of heat necessary for causing evaporation of phenol in the above operation. However, in case where the circulating liquid is heated for the purpose of evaporating a large amount of residual phenol in the flasher system, it is necessary to heat the liquid to an excess degree, so that the above proposal can not be said the best method.
As viewed above, many proposals have been made on the method of harvesting bisphenol A from phenol-containing bisphenol A by evaporating phenol, but none of them is enough satisfactory. Accordingly, the present invention is intended to provide an improved method of obtaining purified bisphenol A by evaporating phenol from phenol-containing bisphenol A.